1. Field of the Invention
The field of this invention relates to the incorporation of continuous graphite fiber reinforcement impregnated with polyamide-imide into injection molded amide-imide parts.
2. Background
Amide-imide polymers and copolymers are a relatively new class of organic compounds known for their solubility in nitrogen-containing organic solvents when in the largely polyamide form. In the past, the major application of these amide-imide polymers has been as wire enamels. This is illustrated in U.S. Pat. Nos. 3,661,832 (1972), 3,494,890 (1970) and 3,347,828 (1967). Amide-imide polymers and copolymers have also been found useful for molding applications as shown in U.S. Pat. Nos. 4,016,140 (1977) and 3,573,260 (1971). U.S. Pat. Nos. 4,136,085 (1979), 4,313,868 (1982), and 4,309,528 (1982) are incorporated herein by reference. These polyamides-imides are known for their outstanding mechanical properties, but they are also difficult to process, as it is particularly difficult to incorporate continuous fiber reinforcement impregnated with the polyamide-imides into injected molded parts made from the polyamide-imides. This difficulty is a consequence of insufficient flow of the polymer. The art has been looking for improvements in the flow and reduction in melt reactivity during fabrication of the polymers, but it is essential that an additive not impair the excellent mechanical properties of the polyamide-imide polymers and copolymers, when forming the continuous fiber inserts for amide-imide molded parts. The ideal flow improving agent for these polymers would be one which plasticizes the polymers during injection molding and crosslinks the polymers and copolymers during the curing or annealing step so that the plasticizing effect would be neutralized by cross-linking.
The general object of this invention is to provide polyamide-imide molded objects reinforced with continuous graphite fiber skins or inserts impregnated with polyamide-imide polymer. A more specific object of this invention is to provide a process for incorporating continuous graphite fiber reinforcement impregnated with polyamide-imide polymer into injection molded parts made from the polyamide-imide. These molded parts are useful where added stiffness and/or strength are critical. Automotive components such as push rods or rocker arms and aircraft components such as fittings or secondary structures are examples. Other objects appear hereinafter.
I have now found that amide-imide polymers and copolymers obtained by reacting a polycarboxylic acid anhydride with one or a mixture of primary diamines can be quickly and readily covered with continuous graphite fiber reinforcement impregnated with amide-imide into injection molded parts by insertion of the prepreg into the mold cavity before injection molding to provide engineering plastic parts with excellent properties. The polymers used in my invention are prepared by reacting an acyl halide derivative of an aromatic tricarboxylic-acid-anhydride with one or a mixture of largely- or wholly-aromatic primary diamines. The resulting products are polyamides wherein the linking groups are predominantly amide groups, although some may be imide groups, and wherein the structure contains free carboxylic acid groups which are capable of further reaction. Such polyamides are moderate molecular weight (7-13,000 as prepared) polymeric compounds having, in their molecule, units of: ##STR1## wherein the free carboxyl groups are ortho to one amide group, Z is an aromatic moiety containing 1 to 4 benzene rings or lower-alkyl-substituted benzene rings, R.sub.1, R.sub.2 and R.sub.3 are the same for homopolymers and are different for copolymers and are divalent wholly- or largely-aromatic hydrocarbon radicals. These hydrocarbon radicals may be a divalent aromatic hydrocarbon radical of from 6 to about 10 carbon atoms, or two divalent aromatic hydrocarbon radicals each of from 6 to about 10 carbon atoms joined directly or by stable linkages, such as --O--, methylene, --CO--, --SO.sub.2 --, --S--; for example, --R'--O--R'--, --R'--CH.sub.2 --R'--, --R'--CO--R', --R'--SO.sub.2 --R'-- and --R'--S--R'--.
Said polyamides are capable of substantially complete imidization by heating, by which they form the polyamide-imide structure having, to a substantial extent, recurring units of: ##STR2## wherein one carbonyl group is meta to and one carbonyl group is para to each amide group and wherein Z, R.sub.1, R.sub.2 and R.sub.3 are defined as above. Typical copolymers of this invention have up to about 50 percent imidization prior to heat treatment, typically about 10 to about 40 percent.
The polyamide-imide copolymers are prepared from an anhydride-containing substance and a mixture of wholly- or partially-aromatic primary diamines. Usually the anhydride-containing substance is an acyl halide derivative of the anhydride of an aromatic tricarboxylic acid which contains about 1 to about 4 benzene or lower-alkyl-substituted benzene rings and wherein two of the carboxyl groups are ortho to one another. More preferably, the anhydride-containing substance is an acyl halide derivative of an acid anhydride having a single benzene or lower-alkyl-substituted benzene ring, and most preferably, the substance is the acyl chloride derivative of trimellitic acid anhydride (4-TMAC).
We can use a single diamine but usually the mixture of diamines contains two or more, preferably two or three, wholly- or largely-aromatic primary diamines. More particularly, they are wholly- or largely-aromatic primary diamines containing from 6 to about 10 carbon atoms or wholly- or largely-aromatic primary diamines composed of two divalent aromatic moieties of from 6 to about 10 carbon atoms, each moiety containing one primary amine group, and the moieties are linked directly or through, for example, a bridging --O--, --S--, --SO.sub.2 --, --CO--, or methylene group. When three diamines are used, they are preferably selected from the class composed of: ##STR3## said X being an --O--, --CH.sub.2 --, or --SO.sub.2 -- group. More preferably, the mixture of aromatic primary diamines is in the one-component or two-component system and is composed of meta-phenylenediamine and p,p'-oxybis(aniline) and meta-phenylenediamine, or p,p'-sulfonylbis(aniline) and p,p'-methylenebis(aniline). Most preferably, the mixture of primary aromatic diamines contains meta-phenylenediamine and p,p'-oxybis(aniline). In the one-component system, the preferred diamines are oxybis(aniline) or meta-phenylenediamine (MPDA). The aromatic nature of the diamines provides the excellent thermal properties of the homopolymer and/or copolymers while the primary amine groups permit the desired imide rings and amide linkages to be formed.
Usually, the polymerization or copolymerization is carried out in the presence of a nitrogen-containing organic polar solvent such as N-methylpyrrolidone (NMP), N,N-dimethylformamide and N,N-dimethylacetamide. The reaction should be carried out under substantially anhydrous conditions and at a temperature below about 150.degree. C. Most advantageously, the reaction is carried out from about 20.degree. to about 50.degree. C.
The reaction time is not critical and depends primarily on the reaction temperature. It may vary from about 1 to about 24 hours, with about 2 to about 4 hours at about 30.degree. to about 50.degree. C. preferred for the nitrogen-containing solvents.
The polyamide-imide polymers are condensation products of trimellitic anhydride and various aromatic diamines, which when polymerized, form a poly(amide-imide) polymer and water. Like all condensation polymers, amide-imide (A-I) resins must be dried prior to processing; however, during plasticization, most polymerization can occur resulting in addition H.sub.2 O to be liberated from the polymer. This moisture can cause problems during molding and cure and must be removed from the part without affecting its shape. To protect the molded part integrity, prolonged cure times are required. When the cure times are shortened, internal foaming occurs resulting in an undesirable dimensional change in the final part as well as internal voids. Foaming can also occur during post curing, thus elaborate post cure cycles are required, especially in thick wall parts. Some of these post cure cycles are as long as 30-40 days with sequential temperature increments from about 300.degree. F. to about 500.degree. F.
Post curing is necessary for our polyamide-imide resins to enhance the glass transition temperature (Tg), and thus properties, by imidizing the molded part at 500.degree. F. while increasing its molecular weight. The lower temperatures are necessary to liberate the internal moisture from the polyamide-imide parts without causing cracking or foaming while raising its Tg.
If a polyamide-imide part is thermally shocked at about 500.degree. F., distortion and/or catastrophic fractures can occur due to the internal pressures generated from the super-heated steam on the uncured polyamide-imide matrix. This is dependent on the part thickness and the amount of H.sub.2 O generated during fabrication.
The injection molding of the polyamide-imides is carried out by injecting the polyamide-imides into a mold maintained at a temperature of about 350.degree. F. to about 450.degree. F., in which continuous graphite fiber impregnated with polyamide-imide polymer has been inserted into the mold cavity. In this process, a 15 to 30 second cycle is used with a barrel temperature of about 580.degree. to about 640.degree. F. The injection molding conditions are given in Table 1.
TABLE 1 ______________________________________ Mold Temperature 350.degree. to 450.degree. F. Injection Pressure 17,000 to 23,000 psi and held for 3 to 7 seconds Back Pressure 500 psi Cycle Time 15 to 30 seconds Extruder: Nozzle Temperature 610.degree. to 650.degree. F. Barrels: Front Heated to 580.degree. to 640.degree. F. Screw Speed 20 to 50 revolutions/minute ______________________________________